Thioxanthone and derivatives thereof are recognized as useful intermediates in the preparation of pharmaceuticals and fine chemicals. Such compounds are also useful as photoinitiators or activators for the photopolymerization of unsaturated compounds, which cure or crosslink upon exposure to radiation. These compounds in turn are useful in the production of photocurable surface coatings and inks. Examples of commercially available thioxanthone derivatives useful as photoinitiators include 2-chlorothioxanthone (CTX) and mixtures of 2- and 4-isopropylthioxanthone (ITX).
Conventionally, thioxanthone and its derivatives are prepared by the substitution and cyclization of thiosalicylic acid (TSA) or dithiosalicylic acid (DTSA) with unsubstituted or corresponding substituted aromatic compounds, such as cumene or chlorobenzene, in a concentrated sulfuric acid medium. See, for example, J.O.C. 24, 1914-1916 (1959); J.A.C.S. 74, 4296-4309 (1952); J.C.S. 97, 197; J.C.S. 97, 1290-1299; J.C.S. 1911, 640-649, 1353-1358, 2046-2051; and J.C.S. 1910, 1290-1299.
Known techniques for preparing thioxanthone and derivatives thereof, including isopropylthioxanthone, are not completely satisfactory. These processes can result in low yields of the desired product or result in a mixture of by-products which are difficult to separate by conventional techniques. In addition to low isolated yields, typically large quantities of concentrated sulfuric acid are used and thereafter diluted with water or neutralized with base to free the product during the isolation steps. The disposal of such large amounts of sulfuric acid can be problematic, and the recovery and reuse of spent acid is difficult and costly. Further, filtration of the thioxanthone product from the highly acidic medium can be extremely slow as a result of difficult and time-consuming washing and drying steps. These processing complications can, in turn, adversely affect the quality and cost of the product.
Efforts have been made to address low yields and complicated separation techniques. However, these attempts have resulted in poor quality products having an undesirable dark color. Because thioxanthone and derivatives thereof are widely used as photoinitiators in the ultraviolet ("UV") curing industry, the resultant product should preferably have a very pale yellow to white color and contain minimal or no organic or inorganic insolubles.
Further, in the UV curing industry, advantageously the photoinitiator is readily dissolved in the photopolymerizable systems, which are typically liquid. However, thioxanthone derivatives prepared according to conventional techniques, and in particular ITX, typically are crystalline or powdered solids with low solubility in organic solvents and photopolymerizable systems. Dissolving the compound or preparing a dispersion thereof can, however, add manufacturing steps, increase labor costs, and produce unstable solutions which can polymerize unexpectedly during extended stirring times and heating.